<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
</head>
<body>
<script>
    var a;
    var b = null;
    console.log(a);
    console.log(a==b);
    console.log(a===b);
    console.log(b+10);
    console.log(a+10);
</script>
</body>
</html>